A known trigger pulse amplifier has a switching circuit with a reference value such that the switching threshold is triggered by the second, i.e., trailing, half wave of the input signal. Accordingly, the leading edge of the output signal occurs only sometime after the occurrence of the zero crossing point of the input signal. This delay depends on both the amplitude as well as the duration of the input signal and may vary in relatively wide domains, in particular if the input signal is derived from the rotational speed of a crankshaft. It is known to attempt compensation of these fluctuations by shifting the threshold of the switch but this compensation is found to be insufficient to hold the leading edge of the output signal within very narrow limits with respect to the zero crossing point of the input signal. Such narrow limits are required however in order to perform exact measurements. It is found that the duration of the output signal is substantially proportional to the duration of the input signal so that the position of the trailing edge of the output signal will depend, for example, on the frequency of the input signals, i.e., the rotational speed of the generating shaft. Accordingly, the occurrence of the trailing edge of the output signal is not suitable for determining the exact angular position of the rotating shaft.